SQL Query Construction

Viasat tracks satellite internet usage per subscriber to enforce data caps. A software engineer is reviewing the query below, written to flag subscribers who've gone over their plan's cap this billing cycle, ahead of adding it to a nightly job. The data_usage table holds roughly 6 billion rows across all terminals and only has a primary key index today.

CREATE TABLE subscribers (
    subscriber_id  BIGINT PRIMARY KEY,
    plan_id         INT,
    signup_date      DATE
);

CREATE TABLE service_plans (
    plan_id       INT PRIMARY KEY,
    plan_name      TEXT,
    data_cap_gb     INT
);

-- data_usage: ~6 billion rows, one per data session, only usage_id is indexed
CREATE TABLE data_usage (
    usage_id       BIGINT PRIMARY KEY,
    subscriber_id  BIGINT REFERENCES subscribers(subscriber_id),
    session_start   TIMESTAMP,
    bytes_used       BIGINT
);

SELECT
    s.subscriber_id,
    sp.plan_name,
    (SELECT SUM(du.bytes_used) FROM data_usage du
       WHERE du.subscriber_id = s.subscriber_id
         AND du.session_start >= '2024-09-01') / 1073741824.0 AS gb_used,
    sp.data_cap_gb
FROM subscribers s
JOIN service_plans sp ON sp.plan_id = s.plan_id
WHERE (SELECT SUM(du2.bytes_used) FROM data_usage du2
         WHERE du2.subscriber_id = s.subscriber_id
           AND du2.session_start >= '2024-09-01') > sp.data_cap_gb * 1073741824
ORDER BY gb_used DESC;

How would you write this SQL query, and what is the size and columns of the table that make it slow?